Method and system for pre-ignition control

ABSTRACT

Methods and systems are provided for mitigating engine pre-ignition based on a feed-forward likelihood of pre-ignition and feedback from a pre-ignition event. In response to an indication of pre-ignition, a cylinder may be enriched while an engine load is limited. The enrichment may be followed by an enleanment to restore exhaust catalyst feed-gas oxygen levels. The mitigating steps may be adjusted based on engine operating conditions, a pre-ignition count, as well as the nature of the pre-ignition.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 12/851,428 filed Aug. 5, 2010, the entire contents of which areincorporated herein by reference for all purposes.

FIELD

The present description relates generally to methods and systems forcontrolling a vehicle engine in response to pre-ignition detection.

BACKGROUND/SUMMARY

Under certain operating conditions, engines that have high compressionratios, or are boosted to increase specific output, may be prone to lowspeed pre-ignition combustion events. The early combustion due topre-ignition can cause very high in-cylinder pressures, and can resultin combustion pressure waves similar to combustion knock, but withlarger intensity. Strategies have been developed for prediction and/orearly detection of pre-ignition based on engine operating conditions.Additionally, following detection, various pre-ignition mitigating stepsmay be taken.

The inventors herein have recognized that not all cylinder pre-ignitionevents are the same, and that pre-ignition mitigation steps may need tobe adjusted based on the nature of the pre-ignition event, as well asthe pre-ignition history of the cylinder. For example, mitigating stepsused for a cylinder with sporadic pre-ignition may not be as effectivefor a cylinder with recurrent pre-ignition. In other words, a moreaggressive approach to pre-ignition mitigation may be required duringsome pre-ignition events as compared to other pre-ignition events.

Thus, in one example, the issue may be addressed by a method ofoperating an engine comprising, in response to intermittent pre-ignitionin a cylinder, enriching the cylinder, and limiting engine load by afirst amount, and in response to persistent pre-ignition in thecylinder, enriching the cylinder, and limiting engine load by a secondamount, greater than the first amount.

In one example, in response to an indication of cylinder pre-ignition,an engine controller may update a cylinder pre-ignition history. Theindication of pre-ignition may be based on engine operating conditionssuch as knock intensity (as determined by a knock sensor), a crankshaftacceleration (as determined by a crankshaft sensor), spark plusionization, and/or based on changes in cylinder pressure as a result ofabnormal cylinder combustion events. The cylinder pre-ignition historymay include, for example, a cylinder pre-ignition count, including anumber of cylinder pre-ignition events that have occurred over thelifetime of the cylinder's operation, as well as a number of cylinderpre-ignition events that have occurred over the current engine cycle.The history may further include a number of consecutive pre-ignitionevents in the cylinder. In one example, in response to a plurality ofdiscrete pre-ignition events over a plurality of consecutive cylindercombustion events (that is, a number of consecutive cylinderpre-ignition events being less than a threshold), an engine controllermay determine intermittent pre-ignition in the cylinder, and may adjustthe mitigating steps accordingly. In another example, in response to aplurality of continuous pre-ignition events over the plurality ofconsecutive cylinder combustion events (that is, a number of consecutivecylinder pre-ignition events exceeding the threshold), the enginecontroller may determine persistent pre-ignition in the cylinder, andmay adjust the mitigating steps accordingly.

For example, in response to the indication of intermittent pre-ignitionin the cylinder, the controller may immediately enrich the cylinder andlimit the engine load by a first amount, while in response to theindication of persistent pre-ignition in the cylinder, the controllermay immediately enrich the cylinder by a second, larger amount. In bothcases, enriching the cylinder may include operating the cylinder at anair-to-fuel ratio richer than stoichiometry for a given duration. Byenriching the cylinder in response to an occurrence of pre-ignition, animmediate cylinder air charge cooling effect may be achieved that mayreduce the occurrence of further abnormal combustion events. Thesimultaneous limiting of engine load, for example by reducing air flow,can further assist in reducing the occurrence of additional pre-ignitionevents. However, the effect of load limiting on pre-ignition may bedelayed until a stable air flow is reached.

The enrichment in response to the intermittent pre-ignition may differfrom the enrichment in response to the persistent pre-ignition. Forexample, the enrichment in response to the intermittent pre-ignition maybe less rich and/or for a shorter duration while the enrichment isresponse to the persistent pre-ignition may be more rich and/or for alonger duration. The engine load limiting may be similarly adjusted. Assuch, limiting an engine load may include reducing the air flow byadjusting one or more of a throttle opening, engine boost, cam timing,valve timing, waste-gate timing, etc. Thus, in one example, in responseto the intermittent pre-ignition, throttle opening and boost may bereduced by a smaller amount, while in response to the persistentpre-ignition, throttle opening and boost may be reduced by a largeramount. In another example, the camshaft timing may be adjusted by alarger amount in response to persistent pre-ignition and by a smalleramount in response to intermittent pre-ignition.

In one example, the load limiting may be synchronized with theenrichment by performing the load limiting at a ramp-in rate that iscoordinated with the enrichment operation. For example, the ramp-in ratemay be adjusted such that ramping in of the limited load is completedconcurrent to completion of the enrichment.

In another example, an engine may include a first and second group ofcylinders. Herein, in response to intermittent pre-ignition in acylinder in the first group of cylinders, the controller may enrich thegiven cylinder and limit an engine load of all cylinders in the firstgroup, and not the second group, for example by adjusting a cam timingof first group but not the second group. In comparison, in response topersistent pre-ignition in a cylinder in the first group, the controllermay enrich the given cylinder and limit an engine load of all cylindersin the first and second group. In one example, this may include limitingan engine load of all cylinders in the first and second group by thesame (larger) amount. In another example, this may include limiting anengine load of all cylinders in the first group by a larger amount whilelimiting an engine load of all cylinders in the second group by asmaller amount.

Similarly, the enrichment profile for other cylinders of the differentgroups may be adjusted differently based on the nature of thepre-ignition. For example, in response to persistent pre-ignition in acylinder in the first group, the given cylinder may be enriched morerich and/or for a longer duration, while other cylinders in the firstgroup are also enriched, but for a shorter duration and/or less rich. Inanother example, cylinders in the second group may also be enriched inresponse to the persistent pre-ignition to reduce the likelihood offurther engine pre-ignition events. In comparison, in response tointermittent pre-ignition in a cylinder in the first group, only thegiven cylinder may be enriched. Still other combinations may bepossible.

Further still, in addition to the enrichment and load limiting, cylinderspark timing may be advanced by an amount. Specifically, spark may beadvanced, relative to the spark timing at the time of pre-ignitiondetection, towards MBT. The amount of spark advance may be adjustedbased on the current engine speed, the enrichment, and/or the nature ofthe pre-ignition. Thus, as the degree of richness and/or duration ofenrichment increases, the amount of spark advance may be increased.Further, a higher amount of spark advance may be used in response topersistent pre-ignition while a smaller amount of spark advance may beused in response to intermittent pre-ignition. Since the cylinder may bemore tolerant to spark advance due to the richer than stoichiometryair-to-fuel ratio during the enrichment, spark advance may beadvantageously used in conjunction with the enrichment to maintain IMEPunder the rich conditions of the cylinder.

The updated pre-ignition history (including the updated pre-ignitioncount) may also be used to determine a likelihood of pre-ignition in acylinder. For example, at the onset of engine operation, a controllermay determine a feed-forward likelihood of pre-ignition based on theengine's operating conditions and further based on the pre-ignitionhistory of the engine's cylinders, and accordingly may preemptivelylimit an engine load based on the likelihood of pre-ignition. The abovedescribed pre-ignition mitigating steps (enrichment and further loadlimiting) may then be applied in response to the indication ofpre-ignition, and the nature of the pre-ignition.

In this way, by adjusting pre-ignition mitigating steps based on thenature of the pre-ignition, pre-ignition related issues may be betteraddressed, and further occurrences of pre-ignition may be reduced.Specifically, by responding more aggressively to persistent pre-ignitionand less aggressively to intermittent pre-ignition, engine pre-ignitionmay be better addressed.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example combustion chamber.

FIG. 2 shows a high level flow chart for addressing pre-ignition basedon a feed-forward likelihood of pre-ignition, as well as based onpre-ignition feedback.

FIG. 3 shows a high level flow chart for limiting an engine load inresponse to a feed-forward likelihood of engine pre-ignition.

FIG. 4 shows a high level flow chart for updating a pre-ignition count,and further limiting an engine load in response to an indication ofpre-ignition.

FIG. 5 shows a schematic depiction of a pre-ignition mitigation routine.

FIG. 6 shows a high level flow chart for executing a fuel injectionoperation to address pre-ignition, according to the present disclosure.

FIG. 7 shows a high level flow chart for adjusting an enrichment profileand a load limiting in a cylinder, bank, or engine, based on apre-ignition count and nature of pre-ignition.

FIGS. 8-9 show example fuel injection operations, according to thepresent disclosure.

DETAILED DESCRIPTION

The following description relates to systems and methods for reducingthe risk of abnormal combustion events related to pre-ignition such asin the engine system of FIG. 1. As elaborated herein with reference toFIGS. 2-5, an engine controller may first determine a likelihood ofpre-ignition based on engine operating conditions, and limit an engineload based on the determined likelihood. Then, in response to anindication of pre-ignition, the controller may update a pre-ignitionhistory (including a pre-ignition count), and may further limit theengine load. The controller may further adjust a fuel injection to oneor more engine cylinders to address pre-ignition without degradingexhaust emissions. For example, the controller may be configured toperform a control routine, such as the routine of FIG. 6, to enrich thecylinders for a first duration, to cool the cylinder air-charge andreduce the risk of further abnormal cylinder combustion events. Theenrichment and load limiting may be adjusted based on the engineoperating conditions, the nature of the pre-ignition, a pre-ignitioncount, etc. For example, the controller may perform a routine, such asthe routine of FIG. 7, to increase the richness and duration of theenrichment, and to increase an amount of load limiting, as apre-ignition count increases, and/or as the pre-ignition becomes morefrequent. Following the enrichment, the cylinders may be transitioned toa lean fuel injection profile for a second duration. The enleanment maybe adjusted based on the preceding enrichment so as to return theexhaust oxygen levels within a range wherein exhaust catalyst efficiencyis not degraded. Following the pre-ignition mitigating fuel injectionoperation, the controller may resume stoichiometric fuel injection.Example fuel injection operations are illustrated herein with referenceto FIGS. 8-9. The controller may store details of the pre-ignition eventin the database to improve anticipation of future pre-ignition events.

FIG. 1 depicts an example embodiment of a combustion chamber or cylinderof internal combustion engine 10. Engine 10 may receive controlparameters from a control system including controller 12 and input froma vehicle operator 130 via an input device 132. In this example, inputdevice 132 includes an accelerator pedal and a pedal position sensor 134for generating a proportional pedal position signal PP. Cylinder (hereinalso “combustion chamber’) 14 of engine 10 may include combustionchamber walls 136 with piston 138 positioned therein. Piston 138 may becoupled to crankshaft 140 so that reciprocating motion of the piston istranslated into rotational motion of the crankshaft. Crankshaft 140 maybe coupled to at least one drive wheel of the passenger vehicle via atransmission system. Further, a starter motor may be coupled tocrankshaft 140 via a flywheel to enable a starting operation of engine10.

Cylinder 14 can receive intake air via a series of intake air passages142, 144, and 146. Intake air passage 146 can communicate with othercylinders of engine 10 in addition to cylinder 14. In some embodiments,one or more of the intake passages may include a boosting device such asa turbocharger or a supercharger. For example, FIG. 1 shows engine 10configured with a turbocharger including a compressor 174 arrangedbetween intake passages 142 and 144, and an exhaust turbine 176 arrangedalong exhaust passage 148. Compressor 174 may be at least partiallypowered by exhaust turbine 176 via a shaft 180 where the boosting deviceis configured as a turbocharger. However, in other examples, such aswhere engine 10 is provided with a supercharger, exhaust turbine 176 maybe optionally omitted, where compressor 174 may be powered by mechanicalinput from a motor or the engine. A throttle 20 including a throttleplate 164 may be provided along an intake passage of the engine forvarying the flow rate and/or pressure of intake air provided to theengine cylinders. For example, throttle 20 may be disposed downstream ofcompressor 174 as shown in FIG. 1, or alternatively may be providedupstream of compressor 174.

Exhaust passage 148 can receive exhaust gases from other cylinders ofengine 10 in addition to cylinder 14. Exhaust gas sensor 128 is showncoupled to exhaust passage 148 upstream of emission control device 178.Sensor 128 may be selected from among various suitable sensors forproviding an indication of exhaust gas air/fuel ratio such as a linearoxygen sensor or UEGO (universal or wide-range exhaust gas oxygen), atwo-state oxygen sensor or EGO (as depicted), a HEGO (heated EGO), aNOx, HC, or CO sensor, for example. Emission control device 178 may be athree way catalyst (TWC), NOx trap, various other emission controldevices, or combinations thereof.

Exhaust temperature may be estimated by one or more temperature sensors(not shown) located in exhaust passage 148. Alternatively, exhausttemperature may be inferred based on engine operating conditions such asspeed, load, air-fuel ratio (AFR), spark retard, etc. Further, exhausttemperature may be computed by one or more exhaust gas sensors 128. Itmay be appreciated that the exhaust gas temperature may alternatively beestimated by any combination of temperature estimation methods listedherein.

Each cylinder of engine 10 may include one or more intake valves and oneor more exhaust valves. For example, cylinder 14 is shown including atleast one intake poppet valve 150 and at least one exhaust poppet valve156 located at an upper region of cylinder 14. In some embodiments, eachcylinder of engine 10, including cylinder 14, may include at least twointake poppet valves and at least two exhaust poppet valves located atan upper region of the cylinder.

Intake valve 150 may be controlled by controller 12 by cam actuation viacam actuation system 151. Similarly, exhaust valve 156 may be controlledby controller 12 via cam actuation system 153. Cam actuation systems 151and 153 may each include one or more cams and may utilize one or more ofcam profile switching (CPS), variable cam timing (VCT), variable valvetiming (VVT) and/or variable valve lift (VVL) systems that may beoperated by controller 12 to vary valve operation. The position ofintake valve 150 and exhaust valve 156 may be determined by valveposition sensors 155 and 157, respectively. In alternative embodiments,the intake and/or exhaust valve may be controlled by electric valveactuation. For example, cylinder 14 may alternatively include an intakevalve controlled via electric valve actuation and an exhaust valvecontrolled via cam actuation including CPS and/or VCT systems. In stillother embodiments, the intake and exhaust valves may be controlled by acommon valve actuator or actuation system, or a variable valve timingactuator or actuation system.

Cylinder 14 can have a compression ratio, which is the ratio of volumeswhen piston 138 is at bottom center to top center. Conventionally, thecompression ratio is in the range of 9:1 to 10:1. However, in someexamples where different fuels are used, the compression ratio may beincreased. This may happen, for example, when higher octane fuels orfuels with higher latent enthalpy of vaporization are used. Thecompression ratio may also be increased if direct injection is used dueto its effect on engine knock. In some embodiments, each cylinder ofengine 10 may include a spark plug 192 for initiating combustion.Ignition system 190 can provide an ignition spark to combustion chamber14 via spark plug 192 in response to spark advance signal SA fromcontroller 12, under select operating modes. However, in someembodiments, spark plug 192 may be omitted, such as where engine 10 mayinitiate combustion by auto-ignition or by injection of fuel as may bethe case with some diesel engines.

In some embodiments, each cylinder of engine 10 may be configured withone or more fuel injectors for providing fuel thereto. As a non-limitingexample, cylinder 14 is shown including one fuel injector 166. Fuelinjector 166 is shown coupled directly to cylinder 14 for injecting fueldirectly therein in proportion to the pulse width of signal FPW receivedfrom controller 12 via electronic driver 168. In this manner, fuelinjector 166 provides what is known as direct injection (hereafter alsoreferred to as “DI”) of fuel into combustion cylinder 14. While FIG. 1shows injector 166 as a side injector, it may also be located overheadof the piston, such as near the position of spark plug 192. Such aposition may improve mixing and combustion when operating the enginewith an alcohol-based fuel due to the lower volatility of somealcohol-based fuels. Alternatively, the injector may be located overheadand near the intake valve to improve mixing. Fuel may be delivered tofuel injector 166 from a high pressure fuel system 8 including fueltanks, fuel pumps, and a fuel rail. Alternatively, fuel may be deliveredby a single stage fuel pump at lower pressure, in which case the timingof the direct fuel injection may be more limited during the compressionstroke than if a high pressure fuel system is used. Further, while notshown, the fuel tanks may have a pressure transducer providing a signalto controller 12. It will be appreciated that, in an alternateembodiment, injector 166 may be a port injector providing fuel into theintake port upstream of cylinder 14.

It will also be appreciated that while the depicted embodimentillustrates the engine being operated by injecting fuel via a singledirect injector; in alternate embodiments, the engine may be operated byusing two injectors (for example, a direct injector and a port injector)and varying a relative amount of injection from each injector.

Fuel may be delivered by the injector to the cylinder during a singlecycle of the cylinder. Further, the distribution and/or relative amountof fuel delivered from the injector may vary with operating conditions.Furthermore, for a single combustion event, multiple injections of thedelivered fuel may be performed per cycle. The multiple injections maybe performed during the compression stroke, intake stroke, or anyappropriate combination thereof. Also, as elaborated in FIG. 6, fuel maybe injected during the cycle to adjust the air-to-injected fuel ratio(AFR) of the combustion. For example, fuel may be injected to provide astoichiometric AFR. An AFR sensor may be included to provide an estimateof the in-cylinder AFR. In one example, the AFR sensor may be an exhaustgas sensor, such as EGO sensor 128. By measuring an amount of residualoxygen (for lean mixtures) or unburned hydrocarbons (for rich mixtures)in the exhaust gas, the sensor may determine the AFR. As such, the AFRmay be provided as a Lambda (λ) value, that is, as a ratio of actual AFRto stoichiometry for a given mixture. Thus, a Lambda of 1.0 indicates astoichiometric mixture, richer than stoichiometry mixtures may have alambda value less than 1.0, and leaner than stoichiometry mixtures mayhave a lambda value greater than 1.

As described above, FIG. 1 shows only one cylinder of a multi-cylinderengine. As such each cylinder may similarly include its own set ofintake/exhaust valves, fuel injector(s), spark plug, etc.

Fuel tanks in fuel system 8 may hold fuel with different fuel qualities,such as different fuel compositions. These differences may includedifferent alcohol content, different octane, different heat ofvaporizations, different fuel blends, and/or combinations thereof etc.

As elaborated with reference to FIGS. 2-7, based on engine operatingconditions, and cylinder pre-ignition history, an engine controller maydetermine a likelihood of pre-ignition, and may adjust an engine loadpreemptively. In response to a subsequent occurrence of pre-ignition ina cylinder, the controller may further limit the engine load and adjusta fuel injection in the cylinder for a defined number of subsequentcombustion events, to enrich the cylinder and mitigate the pre-ignition.In one example, the detection of pre-ignition may involve sensingabnormal combustion events and differentiating abnormal combustionevents due to knocking from those indicative of pre-ignition. Forexample, input from an in-cylinder knock sensor and a crankshaftacceleration sensor may be combined to indicate an abnormal combustionevent in the cylinder. The knock sensor may be an accelerometer on theengine block, or an ionization sensor configured in the spark plug ofeach cylinder. Based on the knock sensor signal, such as a signaltiming, amplitude, intensity, frequency, etc., and/or based on thecrankshaft acceleration signal, the controller may identifypre-ignition. For example, pre-ignition may be indicated in response toan earlier, larger, and/or more frequent signal from the knock sensorwhile knock may be indicated in response to a later, smaller, an/or lessfrequent signal from the knock sensor. Additionally, pre-ignition may bedistinguished from knock based on the engine operating conditions at thetime of abnormal combustion detection. For example, abnormal combustiondetected at higher engine speeds and loads may be attributed to knockingwhile those at lower engine speeds and loads may be indicative ofpre-ignition. As such, mitigating actions taken to address knock maydiffer from those taken by the controller to address pre-ignition. Forexample, knock may be addressed using spark retard and EGR. Pre-ignitionaddressing actions are further elaborated herein with reference to FIGS.2-7.

Controller 12 is shown in FIG. 1 as a microcomputer, includingmicroprocessor unit 106, input/output ports 108, an electronic storagemedium for executable programs and calibration values shown as read onlymemory chip 110 in this particular example, random access memory 112,keep alive memory 114, and a data bus. Controller 12 may receive varioussignals from sensors coupled to engine 10, in addition to those signalspreviously discussed, including measurement of inducted mass air flow(MAF) from mass air flow sensor 122; engine coolant temperature (ECT)from temperature sensor 116 coupled to cooling sleeve 118; a profileignition pickup signal (PIP) from Hall effect sensor 120 (or other type)coupled to crankshaft 140; throttle position (TP) from a throttleposition sensor; absolute manifold pressure signal (MAP) from sensor124, cylinder AFR from EGO sensor 128, and abnormal combustion from aknock sensor and a crankshaft acceleration sensor. Engine speed signal,RPM, may be generated by controller 12 from signal PIP. Manifoldpressure signal MAP from a manifold pressure sensor may be used toprovide an indication of vacuum, or pressure, in the intake manifold.

Storage medium read-only memory 110 can be programmed with computerreadable data representing instructions executable by processor 106 forperforming the methods described below as well as other variants thatare anticipated but not specifically listed.

Engine controller 12 may be configured to anticipate pre-ignition basedon engine operating conditions and to limit an engine load based on afeed-forward likelihood of pre-ignition. As elaborated herein withreference to FIG. 3, a stochastic model may be used to determine alikelihood of pre-ignition based on engine operating conditions such asengine manifold pressure, temperature, fuel octane content, and lambda,and further based on an engine's pre-ignition history. The pre-ignitionhistory may be used to determine a pre-ignition count representative ofpre-ignition occurrences over the lifetime of the vehicle, over a givenengine drive cycle, as well as a consecutive number of pre-ignitionoccurrences. In response to a pre-ignition event, the pre-ignitionhistory and pre-ignition count may be updated, and the updatedinformation may be used to adjust the pre-ignition likelihood computedby the stochastic model in a closed-loop fashion. The pre-ignition eventmay itself be indicated based on the input from multiple sensors.Weighting factors may be used to determine a confidence in a signalbeing indicative of a pre-ignition combustion event. Based on theindication of a pre-ignition event, a cylinder fuel enrichment operationmay be immediately performed to provide a faster response topre-ignition, while the engine load may be further limited to provide aslower response to the pre-ignition. By using a faster fuel injectionbased approach and a slower engine load based approach to addresspre-ignition, further occurrences of pre-ignition may be reduced.

Now turning to FIG. 2, an example routine 200 is described foraddressing cylinder pre-ignition using preemptive steps based on afeed-forward likelihood, and reactive steps in response to an occurrenceof pre-ignition.

At 202, engine operating conditions may be determined. These mayinclude, for example, engine speed, torque, engine load, enginetemperature, engine manifold pressure, air temperature, etc. At 204, andas further elaborated with reference to FIG. 3, a feed-forwardpre-ignition likelihood may be determined based on the estimated engineoperating conditions and further based on an engine pre-ignitionhistory. At 206, the engine load may be limited based on thefeed-forward likelihood of pre-ignition. As further elaborated in FIG.3, this may include reducing an amount of aircharge delivered to theengine, and also slowing ramping in the limited engine load so as toreduce the occurrence of sudden abnormal combustion events. Limiting theengine load may include reducing the air flow by decreasing a throttleopening, adjusting a wastegate timing, valve timing, and/or cam timing,or reducing boost.

However, even after limiting engine load, pre-ignition may still occur.At 208, an occurrence (or indication) of cylinder pre-ignition may beconfirmed. The indication of pre-ignition may be based on one or more ofa cylinder pressure, knock intensity, crankshaft acceleration, and sparkplug ionization. If no cylinder pre-ignition occurs, or no indication ofcylinder pre-ignition is determined, the routine may end with the enginebeing operated with the limited load. However, if an indication ofcylinder pre-ignition is confirmed, then at 210, the engine load may befurther limited. The limiting may be based on the pre-ignition feedback,an updated pre-ignition count, and the nature of the detectedpre-ignition. As elaborated herein with reference to FIG. 4,pre-ignition may be indicated based at least on crankshaft accelerationand knock intensity, and accordingly a pre-ignition history, including apre-ignition count, may be updated in the database. As furtherelaborated in FIG. 7, an amount and rate of load limiting may beadjusted based on whether the pre-ignition is of a persistent nature orintermittent nature, whether a threshold number of pre-ignition eventshave occurred, the pre-ignition count, etc. Further limiting the loadmay include further reducing an air flow by reducing a boost provided bya boosting device (such as a turbocharger), reducing a throttle opening,and/or adjusting a camshaft timing of a variable cam timing mechanism tothereby further adjust a valve timing. As such, the load limiting may bea slower response to the pre-ignition occurrence since it may requirethe engine air flow to stabilize.

At 212, also in response to the occurrence of pre-ignition, a fuelinjection based pre-ignition mitigation operation may be performed.Specifically, the cylinder may be enriched to provide a substantiallyimmediate cylinder charge cooling effect to mitigate the pre-ignition.The enrichment profile of the fuel injection, such as a degree ofrichness, an air-to-fuel ratio, and a duration of enriching, may beadjusted based on the indication of pre-ignition, the pre-ignitionfeedback, the updated pre-ignition count, and the nature of thepre-ignition. For example, where the indication of pre-ignition is basedat least on a knock intensity, the enrichment may be adjusted based onthe knock intensity. For example, a degree of richness and duration ofthe enrichment may be increased as the knock intensity (at the time ofpre-ignition indication) increases. As elaborated in FIG. 6, byadjusting the enrichment responsive to the pre-ignition details, thepre-ignition may be addressed more or less aggressively, as required.Following the enrichment, to compensate for potential drop in catalyticconverter efficiency due to low exhaust feed-gas oxygen levels, the fuelinjection operation may further include a subsequent enleanment.Specifically, the cylinder may be enleaned, an enleanment profile of thefuel injection, such as a degree of leanness, an air-to-fuel ratio, anda duration of enleaning, may be adjusted based on the precedingenrichment.

Additionally, or optionally, the engine controller may be configured toadjust an amount of spark advance for the affected cylinder, and/orother cylinders. For example, spark may be advanced by an amount,relative to the spark timing at the time of pre-ignition detection,towards MBT. The amount of spark advance may be adjusted based on one ormore of the engine speed and the enrichment. In one example, as thedegree of richness and/or duration of the enrichment increases, theamount of spark advance may be increased. Since the cylinder may be moretolerant to spark advance due to the richer than stoichiometryair-to-fuel ratio during the enrichment, spark advance may beadvantageously used in conjunction with the enrichment to maintain IMEPunder the rich conditions of the cylinder. In one example, spark advancemay be modified for the whole engine. In another example, spark advancemay be modified for the affected cylinder while spark advance may befrozen for the remaining cylinders. In still another example, sparkadvance may be frozen for the whole engine.

While the depicted example shows the cylinder being enriched and anengine load being limited in response to a similar indication ofpre-ignition, in an alternate embodiment, the enrichment and the loadlimiting may be performed responsive to differing indications ofpre-ignition, the different indications having different thresholds. Forexample, in response to a first indication of pre-ignition in acylinder, the first indication higher than a first threshold, thecylinder may be enriched. In comparison, in response to a secondindication of pre-ignition in the cylinder, the second indication higherthan a second threshold, the cylinder may be enriched and an engine loadof the cylinder may be limited. Herein, the second threshold may behigher than the first threshold. In one example, where the indication ofpre-ignition in a cylinder is based on a knock intensity of thecylinder, cylinder pre-ignition may be addressed by only enriching thecylinder when the knock intensity exceeds a first, lower threshold. Incomparison, cylinder pre-ignition may be addressed by enriching thecylinder and limiting an engine load of the cylinder when the knockintensity exceeds a second, higher threshold.

At 214, the database may be updated with the details of the currentpre-ignition mitigation operation. This may include updating one or morepre-ignition counts, details of the amount of load limiting used toaddress the pre-ignition, details of the enrichment used to address thepre-ignition, as well as an efficacy of the methods used in addressingthe pre-ignition. As such, as an occurrence of pre-ignition in acylinder increases, the propensity of that cylinder to pre-ignite againmay also increase. Thus, updating the database with details of thecylinder pre-ignition, future pre-ignition events may be betteranticipated and better addressed. For example, as the pre-ignition countof a given cylinder increases, the amount of feed-forward load limitingof that cylinder (or bank) may be increased (for example, relative to aprevious cycle). Additionally, in the event of a further occurrence ofpre-ignition in that cylinder, despite the load limiting, the fuelinjection in the cylinder may be made more rich, or may be prolonged fora longer duration. In this way, feed-forward and feedback methods may beused to better anticipate and better address cylinder pre-ignition.

It will be appreciated that along with the pre-ignition mitigatingsteps, additional steps may be taken to preemptively address the NVH andvibrations arising during a pre-ignition event. For example, theengagement of a torque converter clutch and/or a power-shift clutch maybe adjusted in engine speed-load regions where there is a highlikelihood of pre-ignition to reduce the transmission of tactiledriveline vibrations. In one example, an amount of torque converter slipmay be increased during regions where the engine load is close to loadlimits to mitigate the driveline NVH of a pre-ignition event, if such anevent should occur. In one example, the torque converter slip may beadjusted in an open-loop manner. As such, by adjusting the torqueconverter slip, hydraulic damping of the tactile impact of a firstpre-ignition event may be increased, thereby improving the drive qualityfelt by the vehicle operator.

Now turning to FIG. 3, an example routine 300 is described forestimating a likelihood of pre-ignition, based on engine operatingconditions and an engine pre-ignition history. By determining afeed-forward likelihood of pre-ignition, an engine load may be limitedpreemptively based on a cylinder's propensity to pre-ignite, therebyreducing an occurrence of pre-ignition related abnormal combustionevents.

At 302, the routine includes estimating, inferring, and/or measuringmanifold pressure (MAP), manifold aircharge temperature (MCT), air-fuelratio (lambda), fuel octane content, engine speed (and load), and apre-ignition count. In one example, the pre-ignition count may includeat least an engine trip PI count and an engine lifetime PI count. Theengine trip PI count may include an estimate of a total number ofpre-ignition events in the engine over the present trip, or enginecycle. The engine lifetime PI count may include an estimate of the totalnumber of pre-ignition events in the engine over the lifetime of engineoperation. As such, the engine lifetime PI count and the engine trip PIcount may be obtained based on individual cylinder lifetime and trip PIcounts. The PI count may indicate each cylinder's pre-ignition historyand may correlate with each cylinder's propensity to furtherpre-ignition. Thus, based on a combination of each cylinder's PI count,the engine's propensity for pre-ignition may be estimated. As furtherelaborated herein, the pre-ignition history and count of each cylinder,and thus the engine, may be updated at the end of each cycle, and may beused to determine how to adjust an enrichment profile and a loadlimiting in the event of cylinder pre-ignition occurrence.

At 304, the routine may include determining a lambse. In one example,lambse may be determined by comparing the scheduled air-to-fuel ratiowith the stoichiometric air-to-fuel ratio. At 306, a “percenteffectiveness” of pre-ignition (that is, a possibility of pre-ignition)may be determined based on the calculated lambse. In general, atair-to-fuel ratios rich of stoichiometry, the propensity to pre-ignitemay decrease. Similarly, at air-to-fuel ratios lean of stoichiometry,the propensity to pre-ignite may also decrease. However, at air-to-fuelratios slightly lean of stoichiometry, the propensity to pre-ignite mayincrease.

At 308, a high load limit and a low load limit for the engine may bedetermined. In one example, the high load limit may be estimated from ahigh load limit table which uses manifold charge temperature (MCT) andengine speed (Ne) and computes load limits for “ideal” conditions,and/or with a high fuel octane. Similarly, the low load limit may beestimated from a low load limit table which also uses manifold chargetemperature (MCT) and engine speed (Ne) and computes load limits for“compromised” conditions, and/or with a low fuel octane. At 310, a loadclip may be determined by using the “percent effectiveness” to blend theoutputs from the high load and low load limit tables. For example, thepercent effectiveness output by the controller may be a number between 0and 1 and may be used as an interpolation multiplier between thecomputed high load and low load limits. At 312, the determined loadclip, or load limit, may be ramped in slowly so as to reduce torquedisturbances. Specifically, the load clip may be filtered (for example,using a rolling average filter) over time (for example, using a filterconstant) to slowly ramp the determined load clip. The controller maycoordinate the load ramping with the engine's fuel injection operationto reduce torque disturbances. In this way, by determining afeed-forward likelihood of pre-ignition, and by reducing an engine loadand/or air flow based on the likelihood of pre-ignition, the occurrenceof abnormal pre-ignition related combustion events may be reduced.

However, even after limiting the load, and ramping the load slowly,there is potential to have some pre-ignition events based on thereal-time engine operating conditions. Thus, in response to a suddenoccurrence of cylinder pre-ignition, an engine controller may beconfigured to further limit the load and to enrich the cylinder byperforming a fuel injection based pre-ignition mitigation operation. Dueto delays in the engine system, reducing load and/or air flow is arelatively slow responding control mechanism. The delays may beattributed to effects such as manifold filling effects and a timeconstant required for reaching a stable airflow. Thus, the impact ofload reduction on reducing the probability of pre-ignition may bedelayed until the airflow stabilizes. In comparison, enrichment basedadjustments may have a faster impact since an amount of fuel deliveredto the cylinder may be varied from zero (at cylinder shut-off) to richerthan the air-fuel set-point (that is, richer than stoichiometry)substantially immediately. As elaborated with reference to FIG. 6, byimmediately enriching fuel in a cylinder wherein pre-ignition isindicated, cylinder air charge cooling may be immediately enabled,thereby quickly reducing the probability of further abnormalpre-ignition related combustion events in the cylinder.

Now turning to FIG. 4, an example routine 400 is described foridentifying and indicating a pre-ignition related abnormal combustionevent, updating a pre-ignition count, and further limiting an engineload based on the indication of pre-ignition.

At 402, crankshaft acceleration data may be determined, for example,based on the output from a crankshaft acceleration sensor. At 404, thecrankshaft acceleration data may be processed to determine a crankshaftconfidence. As such, the crankshaft confidence may represent a potentialfor pre-ignition based on the crankshaft acceleration data. In oneexample, the controller may use a function receiving inputs from anengine speed and load table along with the crankshaft acceleration datato determine the crankshaft confidence. The function may be populatedsuch that, for a given engine speed and load combination, cells wherethe crankshaft acceleration data has a higher signal to noise ratio willresult in a higher crankshaft confidence number, while cells where thecrankshaft acceleration data is more prone to noise (such as, fromcrankshaft torsional vibrations) will result in a lower crankshaftconfidence number.

At 406, knock intensity may be determined, for example, based on theoutput from a knock sensor. At 408, the knock intensity may be processedto determine a knock confidence representative of a potential forpre-ignition based on the knock data. As such, the knock confidence maybe determined in a manner similar to the crankshaft confidence.Specifically, the controller may use a function receiving inputs from anengine speed and load table along with the knock data to determine theknock confidence. The function may be populated such that, for a givenengine speed and load combination, cells where the knock intensity has ahigher signal to noise ratio (for example, higher than a threshold) willresult in a higher knock confidence number, while cells where the knockdata is more prone to noise (for example, higher mechanical enginenoise) will result in a lower knock confidence number.

At 410, the knock confidence and crankshaft confidence outputs may becombined, and at 412, the combined output may be compared to athreshold. In one example, the knock confidence and the crankshaftconfidence may be given equal weightage. In another example, a weightingfactor of the knock confidence may be different from the weightingfactor of the crankshaft confidence, the weightages varied based onoperating conditions. For example, at higher engine speeds, whereknocking may be more prevalent, the knock confidence may be given higherweightage. If the combined output is greater than the threshold, then at418, it may be determined that a pre-ignition event has occurred and apre-ignition flag (PI_flag) may be set to true. The output of thepre-ignition flag may then be integrated into at least two differentintegrators. At 420, the output may be integrated on a trip pre-ignitioncount integrator (including a cylinder trip pre-ignition countintegrator and an engine trip pre-ignition integrator) that counts thenumber of pre-ignition events in the current drive cycle (for thecylinder and engine respectively). Specifically, the pre-ignition (PI)count on the Trip PI count integrator may be incremented and the updatedcount may be used to adjust a trip load modifier output of theintegrator (load_modifier_trip). At 422, the output may be integrated ona lifetime pre-ignition count integrator (including a cylinder lifetimepre-ignition count integrator and an engine lifetime pre-ignitionintegrator) that counts the number of pre-ignition events in the life ofthe vehicle engine. Specifically, the pre-ignition (PI) count on theLifetime PI count integrator may be incremented and the updated countmay be used to adjust a lifetime load modifier output of the integrator(load_modifier_life).

In comparison, if the combined output at 412 is not greater than thethreshold, then at 414, it may determined that a pre-ignition event hasnot occurred and the pre-ignition flag (PI_flag) may be set to false.The output of the pre-ignition flag may then be integrated into the tripPI count integrator. Specifically, the PI count on the Trip PI countintegrator may be decremented, or may remain unchanged, and the updatedcount may be used to adjust the trip load modifier output of theintegrator.

At 424, the updated load modifier outputs from the trip PI count andlifetime PI count integrators may be used to update the percenteffectiveness of the engine. At 426, based on the updated percenteffectiveness, the load clip may be adjusted. For example, as thepre-ignition count increases, the engine load may be further limited,and the further limited load clip may be ramped in slowly. For example,the updated load clip may be filtered with an updated rolling averagefilter and an updated filter time constant to reduce torque disturbancesand a harsh feel. As such, the further limiting of engine load may beperformed alongside the fuel injection based pre-ignition mitigationoperation. Thus, in one example, an engine controller may be configuredto co-ordinate the fuel injection operation with ramping of the limitedengine load. For example, the ramp-in rate of the limited load may beadjusted based on the duration of the fuel injection enrichment, and aflag may be set when ramping in of the limited engine load is completed.The engine controller may use the flag to stop a pre-ignition mitigatingfuel enrichment operation. In other words, the amount of fuel injectedduring the fast response fuel injection pre-ignition mitigation may bephased in together with the slower response load reduction.

As further elaborated with reference to FIG. 7, at 424, the load clipmay also be adjusted based on the nature of the pre-ignition. Forexample, based on whether the pre-ignition is intermittent or persistentin nature. The nature of the pre-ignition may be inferred from thepre-ignition count. For example, based on a number of consecutivepre-ignition events over a plurality of consecutive cylinder combustionevents, the intermittent or persistent nature of the pre-ignition may bediscerned, and a load limiting and enrichment operation may beaccordingly adjusted. For example, as a number of consecutivepre-ignition events exceeds a threshold, persistent pre-ignition may bedetermined, and the load may be more limited relative to intermittentpre-ignition.

It will be appreciated that while the depicted example adjusts thepre-ignition count, and consequently the pre-ignition mitigatingoperations based on a number of pre-ignition events over a drive cycle,in alternate embodiments, the pre-ignition count may be determined basedon a number of pre-ignition events over a key cycle, a predeterminedamount of time, or a mileage. In one example, the mileage used may bethe total mileage of the vehicle over the lifetime of the vehicle, orover the current trip. In another example, the pre-ignition count may beadjusted based on a mileage since a preceding pre-ignition event. Forexample, a pre-ignition count may be decremented if a singlepre-ignition event is determined to occur after a threshold mileagesince a preceding pre-ignition event. In another example, when multiplepre-ignition events are detected (such as during persistent orintermittent pre-ignition), the count and pre-ignition mitigation(enrichment, load limiting, etc) associated with the detectedpre-ignition event may be reduced as the mileage between successivemultiple pre-ignition events exceeds a threshold.

In this way, by updating a pre-ignition count in response to anoccurrence of pre-ignition and limiting an engine load based on thepre-ignition feedback, pre-ignition may be better addressed.

FIG. 5 shows a schematic representation of feed-forward and feedbacklimiting of an engine load in response to pre-ignition. As such, thefigure is an alternate depiction of the routines of FIGS. 2-4. Method500 comprises, at sub-routine 510 (and as elaborated previously in FIG.4), determining a feed-forward likelihood of pre-ignition and limitingan engine load based on the feed-forward likelihood, to thereby reducean occurrence of a cylinder pre-ignition event. Method 500 furthercomprises, at sub-routine 580, determining and indicating the occurrenceof a cylinder pre-ignition event, and further limiting the engine loadbased on feedback from the occurrence. As such, sub-routine 510 may beperformed under conditions when pre-ignition has not been detected, thatis, in anticipation of pre-ignition. In comparison, sub-routine 580 maybe additionally performed in response to an indication of a pre-ignitionevent.

Sub-routine 510 includes comparing a scheduled air-to-fuel ratio 501(Fntqe_High_vol) with a set air-to-fuel ratio, lambda 502, (such as, astoichiometric air-to-fuel ratio), to determine a lambse 504. The lambsemay then be processed through multiplier 506 to determine percenteffectiveness 508. As such, percent effectiveness 508 may berepresentative of a propensity for pre-ignition, and may be output as anumber between 0 (no likelihood of pre-ignition) and 1 (high risk ofpre-ignition). Percent effectiveness 508 may be used as an interpolationmultiplier between a high load limit 514 and a low load limit 516 todetermine a load clip 518. High load limit 514 (Fntqe_high_ld_eff) maybe calculated using a table of engine load versus engine speed under“ideal” conditions (with high fuel octane). Low load limit 516(Fntqe_low_ld_eff) may be calculated using a similar table of engineload versus engine speed under “compromised” conditions (with low fueloctane). Controller 512 may blend the high speed limit 514 and the lowspeed limit 516 using percent effectiveness 508. In one example,controller 512 may blend the limits using an equation as follows,Tqe_ld_limit_tmp=(tqe_pct_eff_tmp*tq_ld_low_eff)+((1−tqe_pct_eff_tmp)*tq_ld_high_eff),

wherein Tqe_ld_limit_tmp is the load clip in anticipation ofpre-ignition, tqe_pct_eff_tmp is the percent effectiveness in theabsence of a pre-ignition event, tq_ld_low_eff is the low load limit andtq_ld_high_eff is the high load limit.

Load clip 518 may be further processed through filter 520 to generatefiltered load clip 522. Filtered load clip 522 may include a rampingrate for the load so as to reduce torque disturbances. In one exampleload clip 518 may be rolling average filtered with a time constant toobtain filtered load clip 522.

In the event of a pre-ignition event, sub-routine 580 may be performedto update the percent effectiveness with load multipliers, therebyfurther limiting the load clip of sub-routine 510. Sub-routine 580 mayinclude identifying pre-ignition based on the output of a crankshaftacceleration sensor 530 and a knock sensor 540. However, in alternateembodiments, pre-ignition may be identified based on the output of oneor more other sensors. The output of crankshaft acceleration sensor 530may be compared to a threshold to determine a crankshaft pre-ignitioncall (PI_CKP_call) 532. As such, PI_CKP_call 532 may have a value ofeither 0, when the output is below the threshold (that is, no crankshaftdata based pre-ignition called) or a value of 1, when the output isabove the threshold (that is, crankshaft data based pre-ignitioncalled). Similarly, the output of knock sensor 540 may be compared to athreshold to determine a knock pre-ignition call (PI_KNK_call) 542. Assuch, PI_KNK_call may have a value of either 0, when the output is belowthe threshold (that is, no knock data based pre-ignition called) or avalue of 1, when the output is above the threshold (that is, knock databased pre-ignition called).

Crankshaft pre-ignition call 532 may be processed by multiplier 536,based on an engine speed and load table 534, to determine a crankshaftconfidence (CKP_confidence) 538. Multiplier 536 may be populated suchthat cells where the crankshaft acceleration sensor output has a highsignal to noise ratio will result in a crankshaft confidence numbercloser to 1 (that is, a higher confidence of pre-ignition), while cellsthat are more prone to noise (such as, from crankshaft torsionalvibrations) will result in a crankshaft confidence number closer to 0(that is, a lower confidence of pre-ignition). Similarly, knockpre-ignition call 542 may be processed by multiplier 546, based on anengine speed and load table 544, to determine a knock confidence(KNK_confidence) 548. Multiplier 546 may be populated such that cellswhere the knock sensor output has a high signal to noise ratio willresult in a knock confidence number closer to 1 (that is, a higherconfidence of pre-ignition), while cells that are more prone to noise(such as, from mechanical engine noise) will result in a crankshaftknock number closer to 0 (that is, a lower confidence of pre-ignition).The confidence numbers from the crankshaft acceleration method and theknock methods may be combined by adder 550 and compared to a thresholdby controller 552 to determine if there is a pre-ignition event. If thecombined output analyzed at controller 552 is greater than thethreshold, then a pre-ignition event may be confirmed and the datarelated to the pre-ignition event may be used to address thepre-ignition. Specifically, a fuel injection based mitigation operationmay be performed wherein the very next combustion event, and a definednumber of combustion events thereafter, are enriched to reduce thepropensity of pre-ignition.

Further, pre-ignition feedback data may be routed to sub-routine 510wherein it may be used to adjust the filtered load clip of sub-routine510. If the combined output is not greater than the threshold, nopre-ignition event may be confirmed, and an engine controller maycontinue to operate the engine with the (unadjusted) filtered load clipof sub-routine 510.

In this way, sensor information from various sensors may be combined toenable a more robust detection of pre-ignition and to enable a morerobust distinction of pre-ignition related abnormal combustion eventsfrom non pre-ignition related abnormal combustion events (such asmisfires and knocking). Furthermore, by using the output from multiplesensors, shortfalls in a given sensor under certain engine operatingconditions may be overcome by the presence of other sensors, andpre-ignition may be detected even in the presence of degradation in oneof the various sensors.

Upon confirming a pre-ignition event, controller 552 may set apre-ignition flag, and may process the flag data through at least twodifferent integrators including a trip pre-ignition (PI) integrator 558and a lifetime pre-ignition (PI) integrator 560. As such, the trip PIintegrator may count the number of pre-ignition events in the currentdrive cycle (that is, from key-on engine start to key-off), and may bereset at each key-off event. Thus, in response to a pre-ignition eventin the drive cycle, a trip pre-ignition count 554 (PI_count_trip) may beincremented by trip PI integrator 558. In one example, trip pre-ignitioncount 554 may include at least a cylinder trip pre-ignition count foreach cylinder (or each group of cylinders), as well as an overall enginetrip pre-ignition count based on the individual cylinder trippre-ignition counts. If no pre-ignition event occurs over at least aduration of the drive cycle, the trip pre-ignition count may bedecremented. Alternatively, if no pre-ignition event occurs, thepre-ignition count may be maintained unchanged. In comparison, lifetimePI integrator 560 may count the number of pre-ignition events in thelife of the vehicle. Thus, in response to a pre-ignition event in thedrive cycle, a lifetime pre-ignition count 556 (PI_count_life) may beincremented by lifetime PI integrator 560. In one example, lifetimepre-ignition count 556 may include at least a cylinder lifetimepre-ignition count for each cylinder (or each group of cylinders), aswell as an overall engine lifetime pre-ignition count based on theindividual cylinder lifetime pre-ignition counts.

Trip PI integrator 558 may generate a trip load modifier 562(load_modifier_trip) based on the trip pre-ignition count 554 whilelifetime PI integrator 560 may generate a lifetime load modifier 564(load_modifier_life) based on the lifetime pre-ignition count 556. Assuch, trip PI integrator 558 may be configured to generate a loadmodifier to reduce the probability of further pre-ignition occurrencesin the same drive cycle, while lifetime PI integrator 560 may beconfigured to generate a load modifier to counteract changes to thevehicle over time. Consequently, the trip load modifier generated by thetrip PI integrator may be a more aggressive function than the lifetimeload modifier generated by the lifetime PI integrator. In one example,load modifiers 562 and 564 may have values between 0 (less aggressive)and 1 (more aggressive). The load modifiers may then be used to furtheradjust percent effectiveness 508, thereby generating an adjusted loadclip 518, and an adjusted filtered load clip 522. In this way, theengine load may be further reduced based on pre-ignition feedback from adetected pre-ignition event.

Now turning to FIG. 6, a routine 600 is described for adjusting a fuelinjected into a cylinder in response to an indication of pre-ignition.By enriching a cylinder in response to an occurrence of a cylinderpre-ignition event, a cylinder cooling effect may be immediatelyachieved to reduce the risk of further abnormal combustion, and enginedegradation.

At 602, an engine controller may determine a first enrichment profilebased on engine operating conditions and the indication of pre-ignition.As previously elaborated with reference to FIGS. 2-4, the indication ofpre-ignition may include a feed-forward pre-ignition likelihooddetermined based on engine operating conditions, as well as asensor-based (such as, a knock sensor and crankshaft acceleration sensorbased) pre-ignition indication. As further elaborated below withreference to FIG. 7, the enrichment profile may be further based on thepre-ignition count of the cylinder, as well as the nature of thepre-ignition (such as, based on whether the pre-ignition is intermittentor persistent in nature).

The first enrichment profile may include, for example, a first air toinjected fuel ratio (AFR) richer than stoichiometry, a degree ofrichness of the AFR, and a first duration for the rich fuel injectionadjusted based on the engine operating conditions. The first durationmay be, for example, a first number of combustion events. Additionallyor optionally, the enrichment profile may include a rate of enrichment(that is, a rate of change of rich AFR) over the first duration.

In one example, the first enrichment profile may include a longer firstduration and/or a higher degree of richness of the first air-to-fuelratio as an indication of pre-ignition increases. For example, where theindication of pre-ignition is based at least on an in-cylinder pressure,or knock intensity, the adjustment of the profile may include increasingthe first duration and/or increasing the degree of richness of theair-to-fuel ratio as a cylinder pressure, or knock intensity at the timeof pre-ignition detection exceeds a threshold. In another example, thedegree of richness and/or the duration of enrichment may be increased asa cylinder pre-ignition count increases (e.g., exceeds a threshold). Instill another example, the degree of richness and duration may beincreased more in response to persistent pre-ignition, and may beincreased less in response to intermittent pre-ignition.

In one example, the enrichment may also be coordinated with the loadlimiting operation (of FIG. 4). For example, a ramp—in rate for thelimited load may be adjusted based on the first duration of theenrichment and/or the rate of enrichment over the first duration suchthat the enrichment operation and the load ramping in are completedsubstantially simultaneously. In one example, a flag may be set whenramping in of the limited engine load is completed. The enginecontroller may use the flag to stop the enrichment operation accordinglyso that the amount of fuel injected during the fast responsepre-ignition mitigating enrichment operation can be phased in togetherwith the slower response load reduction.

At 604, the controller may fuel and operate the cylinder according tothe first enrichment profile. For example, the cylinder may be operatedwith the first air-to-fuel ratio richer than stoichiometry for the firstduration. In another example, the cylinder may be enriched at a firstrate of enrichment, and the cylinder may be operated with an air-to-fuelratio richer than stoichiometry, the air-to-fuel ratio changing at thefirst rate over the first duration. Herein, by injecting excess fuelinto the cylinder, a charge cooling effect may be obtained and cylinderpeak pressures may be lowered to reduce the risk of further pre-ignitionrelated abnormal combustion events.

However, the excess fuel immediately injected to mitigate thepre-ignition risk may also deplete exhaust feed-gas oxygen and therebyreduce the catalytic efficiency of emission control device catalyticconverters. The excess fuel may also have an adverse impact on exhaustemissions. Thus, to restore the catalytic efficiency of the catalyticconverters, at 606, the controller may determine a second enleanmentprofile based on the first enrichment profile of the precedingenrichment. The second enleanment profile may include, for example, asecond air to injected fuel ratio (AFR) leaner than stoichiometry, adegree of leanness of the AFR, a second duration for the enleanment, anda rate of enleanment, each adjusted based on one or more of the firstair to fuel ratio and the first duration of the first rich fuelinjection profile. The second duration may be, for example, a secondnumber of combustion events. The adjustment may include, for example,increasing the second duration and/or increasing the degree of leannessof the air-to-fuel ratio as one or more of the first duration and adegree of richness of the first air-to-fuel ratio increases.

At 608, after the first duration has elapsed, the controller may fueland operate the cylinder according to the second enleanment profile. Forexample, the cylinder may be operated with the second air-to-fuel ratioleaner than stoichiometry for the second duration. In another example,the cylinder may be enleaned at a second rate of enleanment, and thecylinder may be operated with an air-to-fuel ratio leaner thanstoichiometry, the air-to-fuel ratio changing at the second rate overthe second duration. Herein, by injecting relatively less fuel andrelatively more air into the cylinder, the oxygen depletion effect ofthe preceding enrichment may be compensated for, and catalyticefficiency of the catalytic converter may be restored.

At 610, the exhaust gas oxygen content may be estimated and/or inferred(for example, using an exhaust gas oxygen sensor) and it may bedetermined whether the exhaust gas oxygen content is above a threshold.The threshold may be an oxygen level above which an emission controldevice catalyst can operate with substantial catalytic efficiency. Ifthe oxygen content has been restored to the threshold level, then at614, following the enleanment, the controller may resume operating thecylinder with a third air-to-fuel ratio substantially at stoichiometry.If the oxygen content has not reached the threshold at 610, then at 612,the controller may continue operating the cylinder(s) at the secondair-to-fuel ratio until a desired oxygen level and a desired catalyticefficiency is restored. Once the catalytic efficiency is restored,stoichiometric cylinder fuel injections may resume.

In one example, the second duration and degree of leanness of the secondenleanment profile may be adjusted such that a threshold exhaust oxygenlevel is restored by the end of the second duration. For example, theadjustment may include increasing the second duration and/or increasingthe degree of leanness of the second air-to-fuel ratio as a total amountof exhaust oxygen consumed over the first duration increases. In anotherexample, where the oxygen consumption over the first duration isinferred from an amount of unburned hydrocarbons (HCs) and carbonmonoxide generated, the adjustment may include, increasing the secondduration and/or increasing the degree of leanness of the secondair-to-fuel ratio as a total amount of exhaust unburned HCs producedover the first duration increases. In either case, after the secondduration has elapsed, the controller may resume operating the cylinderwith the third substantially stoichiometric air-to-fuel ratio.

In this way, by enriching a cylinder in response to an indication ofpre-ignition, the charge-cooling effect of the injected fuel may be usedto immediately and rapidly address pre-ignition. By concomitantlylimiting an engine load based on the indication of pre-ignition, furtheroccurrences of pre-ignition may be substantially mitigated.

Now turning to FIG. 7, an example routine 700 is described for adjustingthe fuel injection profiles for the fuel injection based pre-ignitionmitigation operation based on the nature of pre-ignition and/or thepre-ignition count. Specifically, based on a pre-ignition count and anature of the pre-ignition, an aggressiveness with which the enrichmentmay be performed may be adjusted. For example, under some pre-ignitionconditions, the enrichment may be performed less aggressively in thegiven affected cylinder only, while in other pre-ignition conditions,the enrichment may be performed more aggressively and may be extended toother cylinders of the bank or engine.

At 702, it may be confirmed that a cylinder pre-ignition event isdetected. If not, the routine may end. Upon confirmation, at 704, apre-ignition count (such as a cylinder and/or engine pre-ignition count)and a pre-ignition database (including details of previous pre-ignitionevents and pre-ignition mitigating operations) may be updated. Aspreviously elaborated, this may include increasing a pre-ignition count,for example, on a trip pre-ignition counter as well as a lifetimepre-ignition counter. The pre-ignition count may include one or more ofa cylinder trip pre-ignition count, a cylinder lifetime pre-ignitioncount, an engine trip pre-ignition count, an engine lifetimepre-ignition count, a cylinder consecutive pre-ignition count, and anengine consecutive pre-ignition count. The trip pre-ignition counts maybe representative of previous pre-ignition events during the same enginecycle/operation, while the lifetime pre-ignition count may berepresentative of all previous pre-ignition events over the entireduration of vehicle operation.

It will be appreciated that while the depicted example increases thepre-ignition count in response to an occurrence of pre-ignition (over adrive cycle, key cycle, predetermined amount of time, etc.), inalternate embodiments, increasing the pre-ignition count may includeincreasing the pre-ignition count based on a mileage of the engine. Inone example, the mileage used may be the total mileage of the engine, orvehicle (over the lifetime of the vehicle, or over the current trip). Inanother example, the mileage may include a mileage since a precedingoccurrence of pre-ignition in an engine cylinder. For example, theengine pre-ignition count may be increased in response to a mileage ofthe engine, since a preceding occurrence of engine pre-ignition,exceeding a threshold. In another example, a pre-ignition count may beincremented if a single pre-ignition event is determined to occur aftera threshold mileage since a preceding pre-ignition event. In anotherexample, when multiple pre-ignition events are detected (such as duringpersistent or intermittent pre-ignition), the count and pre-ignitionmitigation (enrichment, load limiting, etc) associated with the detectedpre-ignition event may be increased as the mileage between successivemultiple pre-ignition events exceeds a threshold.

At 706, a total number of cylinder pre-ignition events may bedetermined, such as, based on the updated pre-ignition counts. At 708, atotal number of consecutive cylinder pre-ignition events may bedetermined (such as, from a consecutive cylinder pre-ignition count).Herein, it may be determined as to how many of all the pre-ignitionevents that have occurred in the cylinder are consecutive, that is, thefrequency of pre-ignition occurrence in different cylinders. At 710, itmay be determined whether the total number of cylinder pre-ignitionevents is greater than a threshold. That is, it may be determined if apre-ignition count is greater than a threshold. If the total number ofcylinder pre-ignition events is not greater than the threshold, then at712, the given affected cylinder may be enriched according to anenrichment profile based on engine operating conditions and based on thepre-ignition count, as discussed in FIG. 6. In comparison, if athreshold number of pre-ignition events has been exceeded, then at 714,it may determined whether a number of consecutive pre-ignition events(that is, the consecutive pre-ignition count) is also greater than athreshold.

Based on the frequency of pre-ignition occurrence, the nature of thepre-ignition may be determined. In one example, when the number ofconsecutive pre-ignition events at 714 is greater than the threshold,persistent pre-ignition may be determined at 716. That is, persistentpre-ignition may be concluded in response to a plurality of continuous,uninterrupted pre-ignition events over a plurality of consecutivecylinder combustion events. In comparison, when the number ofconsecutive pre-ignition events at 714 is less than the threshold, whilethe total number of pre-ignition events at 710 is greater than athreshold, intermittent pre-ignition may be determined at 722. That is,intermittent pre-ignition may be concluded in response to a plurality ofdiscrete, interrupted pre-ignition events over a plurality ofconsecutive cylinder combustion events.

In an alternate example, persistent pre-ignition may be determined inresponse to a continuous and steady increase in the trip and lifetimepre-ignition count of a cylinder, while intermittent pre-ignition may bedetermined in response to a smaller increase in the trip pre-ignitioncount for a given increase in the lifetime pre-ignition count. In stillanother example, persistent pre-ignition may be determined in responseto a pre-ignition combustion event on each combustion cycle, whileintermittent pre-ignition may be determined in response to apre-ignition combustion event on every other (or more) combustioncycles.

At 724, in response to intermittent pre-ignition in the cylinder, engineload may be limited by a first smaller amount, while at 718, in responseto persistent pre-ignition in the cylinder, engine load may be limitedby a second greater amount. For example, in response to intermittentpre-ignition, boost may be reduced by a (first) smaller amount, throttleopening may be reduced by a (first) smaller amount, or camshaft timingmay be adjusted by a (first) smaller amount. In comparison, in responseto persistent pre-ignition, boost may be reduced by a (second) largeramount, throttle opening may be reduced by a (second) larger amount, orcamshaft timing may be adjusted by a (second) larger amount.

Similarly, the enrichment profiles may be adjusted differently. Forexample, at 726, the enrichment in response to intermittent pre-ignitionmay be made less rich and/or of a shorter duration, while at 720, theenrichment in response to persistent pre-ignition may be made more richand/or for a longer duration. The degree of richness of the enrichment,and/or the duration of the enrichment, as well as the amount of loadlimiting may also be increased as the number of consecutive pre-ignitionevents in the cylinder increases (e.g., exceeds the threshold). That is,persistent pre-ignition may be addressed more aggressively thanintermittent pre-ignition.

Further, based on the nature of the pre-ignition and the pre-ignitioncount, the enrichment and load limiting may be extended to othercylinders of the engine. In one example, the pre-igniting cylinder maybe located on a first cylinder group (or bank) of the engine. Herein, inresponse to persistent pre-ignition in the cylinder, limiting engineload may include limiting an engine load of all cylinders on the firstgroup more than the cylinders on the second group. In one example, thismay be achieved by adjusting a camshaft timing of the first group morethan the second group. In comparison, in response to intermittentpre-ignition in the cylinder, only the engine load of the first groupand not the second group may be limited, for example, by maintaining thecamshaft timing of the second group while adjusting the camshaft timingof the first group. In another example, in response to persistentpre-ignition, all engine cylinders may be limited by a larger amount,while only the cylinders of the first group may be limited by a smalleramount in response to intermittent pre-ignition.

Similarly, in response to persistent pre-ignition in a cylinder in thefirst group, the cylinders of the first group, but not the second groupmay be enriched, the cylinders of the first group may be enriched more(e.g., more rich and/or for a longer duration) than the cylinders of thesecond group, or all engine cylinders may be enriched equally rich (atthe higher amount). In still another example, in response to persistentpre-ignition in a cylinder, all engine cylinders may be enriched, thepre-ignition cylinder enriched more (more rich and/or for a longerduration), and an enrichment of all the other cylinders adjusted basedon their firing order (e.g., the enrichment of a cylinder firingimmediately after the affected cylinder adjusted to be more rich than acylinder firing later).

As previously elaborated, in addition to the enrichment and loadlimiting, cylinder spark timing may be advanced by an amount, relativeto the spark timing at the time of pre-ignition detection, towards MBT.In addition to engine speed and enrichment, the amount of spark advancemay also be adjusted based on the nature of the pre-ignition. Forexample, spark timing may be advanced by a larger amount in response topersistent pre-ignition, while spark timing may advanced by a smalleramount in response to intermittent pre-ignition.

In this way, persistent pre-ignition may be addressed more aggressivelythan intermittent pre-ignition. While the depicted example illustratesadjusting the enrichment and load-limiting differently based on theintermittent or persistent nature of pre-ignition, still otherintermediate pre-ignition states may also be possible that are based onthe rate of change in pre-ignition count. For example, a more aggressiveapproach may be used in response to a faster increase in pre-ignitioncount while a less aggressive approach may be used in response to aslower increase in pre-ignition count.

Still other adjustments to the enrichment and load limiting profile maybe possible based on the pre-ignition count. As previously discussedwith reference to FIG. 6, the enrichment of a cylinder may be increasedas the pre-ignition count increases and exceeds a threshold. In oneexample, during a first condition, with a first, lower number of(previous) pre-ignition events, operation of a first cylinder may beadjusted (e.g., enriched) in response to an indication of pre-ignitionin the first cylinder. In comparison, during a second condition, with asecond, higher number of (previous) pre-ignition events, operation of afirst and a second cylinder may be adjusted (e.g., enriched) in responseto an indication of pre-ignition in the first cylinder. In some example,the enrichment of the first cylinder in the first condition may be lessrich and/or for a shorter duration than the enrichment of the secondcylinder in the second condition. Similarly, during the first condition,a load of the first cylinder may be limited by a first smaller amountwhile during the second condition, the load of the first and secondcylinder may be limited by a second, higher amount.

The adjustment may also vary differently based on different pre-ignitioncounts. For example, an engine load limiting and enrichment in responseto the cylinder trip pre-ignition count may be more than the loadlimiting in response to the cylinder lifetime pre-ignition count. Thatis, pre-ignition issues over an engine cycle may be addressed moreaggressively than overall engine pre-ignition issues, to curb furtherpre-ignition events over the same engine cycle. In another example,engine load limiting and enrichment in response to the engine trippre-ignition count may be more than the load limiting in response to thecylinder trip pre-ignition count.

In still another example, when the affected cylinder is in a first groupof cylinders of the engine, all engine cylinders may be enriched whenthe consecutive pre-ignition count is greater than a first, higherthreshold, while engine cylinders of the first group, but not the secondgroup, may be enriched when the consecutive pre-ignition count isgreater than a second, lower threshold. Similarly, an engine load of theall engine cylinders may be limited when the consecutive pre-ignitioncount is greater than the first, higher threshold, while the an engineload of the first group of cylinders may be limited more than the secondgroup, when the consecutive pre-ignition count is greater than thesecond, lower threshold. Still other combinations may be possible.

In this way, the enrichment and load limiting in an engine in responseto a cylinder pre-ignition event may be adjusted not only based onengine operating conditions, but also based on a cylinder's pre-ignitionhistory, a pre-ignition count, and a nature of the pre-ignition. In thisway, the propensity for further pre-ignition in the affected enginecylinder, as well as other engine cylinders, may better anticipated andpre-ignition may be better addressed.

Example enrichment and enleanment profiles, as described previously, arenow explained in the example fuel injection operations of FIGS. 8-9.

First turning to FIG. 8, map 800 depicts a first example pre-ignitionmitigating operation according to the present disclosure. An air-to-fuelratio (AFR) of the injected fuel mixture is shown along the y-axis whiletime is depicted over the x-axis. As shown, before t1, the fuel mixtureinjected inside a cylinder may be substantially at stoichiometry. At t1,in response to pre-ignition indication 801, the affected cylinder may beoperated with a first air-to-fuel ratio AFR1 richer than stoichiometryfor a first duration 804. The degree of richness 806 of the injectionand the first duration 804 may be adjusted based on engine operatingconditions at the time of the pre-ignition indication 801. After thefirst duration has elapsed, at t2, a controller may be configured todetermine an amount of excess fuel injected over the first enrichmentoperation. As such, the amount of excess fuel may be computed as an area808 under the curve of the first enrichment operation. That is, area 808may represent an integral of excess fuel and unburned HCs from theenrichment.

At t2, the cylinder may be transitioned from the first air-to-fuel ratioAFR1 to a second air-to-fuel ratio AFR2 leaner than stoichiometry for asecond duration 810. One or more of the degree of leanness 812 of thelean fuel injection and the second duration 810 may be adjusted based onone or more of the first duration 804 and the degree of richness 806.For example, the degree of leanness 812 and/or the second duration 810of the second enleanment operation may be increased as the firstduration 804 and the degree of richness 806 of the first enrichmentoperation increases. The degree of leanness 812 and the second duration810 may be selected such that an amount of excess exhaust oxygengenerated over the lean fuel injection operation may compensate for theexcess fuel injected over the rich fuel injection operation. As such,the amount of excess oxygen may be computed as an area 814 under thecurve of the second lean fuel injection. That is, area 814 may be anintegral of the excess oxygen from the enleanment operation. Thus, thesecond duration 810 may be adjusted based on the exhaust gas contentsuch that the cylinder is continued to be operated at the secondair-to-fuel ratio AFR2 until the exhaust gas oxygen content is returnedabove a threshold.

At t3, after the second duration 810 has elapsed, and the exhaust gasoxygen content has been restored above the threshold value, the cylindermay be transitioned back from the second air-to-fuel ratio AFR2 to anair-to-fuel ratio substantially at stoichiometry.

Now turning to FIG. 9, map 900 depicts a second example pre-ignitionmitigating operation according to the present disclosure. Herein, beforet1, the fuel mixture injected inside the cylinder may be substantiallyat stoichiometry. At t1, in response to pre-ignition indication 901, theaffected cylinder may be enriched for a first duration 904 at a firstrate of enrichment 905 over the first duration. Specifically, thecylinder may be operated at an air to injected fuel ratio richer thanstoichiometry, and the degree of richness 906 of the air to injectedfuel ratio may be varied over the first duration 904 such thatair-to-fuel ratio AFR1 richer than stoichiometry is attained before theend of first duration 904. In one example, as depicted, the rate ofenrichment 905 may be adjusted such that a degree of richness mayincrease as a number of combustion events since pre-ignition increases.In an alternate example, the rate of enrichment 905 may be adjusted suchthat a degree of richness may decrease as a number of combustion eventssince pre-ignition increases. After the first duration has elapsed, att2, the cylinder may be configured to determine an amount of excess fuelinjected over the first enrichment operation. As such, the amount ofexcess fuel generated over the enrichment may be computed as an area 908under the curve of the enrichment operation. That is, area 908 mayrepresent an integral of excess fuel and unburned HCs from theenrichment.

At t2, the cylinder may be transitioned from an air-to-fuel ratio richerthan stoichiometry to an air-to-fuel ratio leaner than stoichiometry fora second duration 910. The second duration 910 of enleanment may bebased on the first duration 904 of the preceding enrichment.Specifically, the affected cylinder may be enleaned at a second rate ofenleanment 915 over the second duration 910. Herein, the cylinder may beoperated at an air to injected fuel ratio leaner than stoichiometry, andthe degree of richness 912 of the air to injected fuel ratio may bevaried over the second duration 910 such that air-to-fuel ratio AFR2leaner than stoichiometry is attained before the end of second duration910. One or more of the degree of leanness 912, rate of enleanment 915,and second duration 910 may be adjusted based on one or more of thefirst degree of richness 906, rate of enrichment 905, and first duration904 of the preceding enrichment. For example, the degree of leanness 912and/or the second duration 910 of the enleanment operation may beincreased as the first duration 904 and the degree of richness 906 ofthe preceding enrichment increases. In one example, as depicted, therate of enleanment 915 may be adjusted such that a degree of leannessmay increase as a number of combustion events since pre-ignitionincreases. In an alternate example, the rate of enleanment 915 may beadjusted such that a degree of leanness may decrease as a number ofcombustion events since pre-ignition increases. The degree of leanness912 and the second duration 910 may be selected such that an amount ofexcess exhaust oxygen generated over the enleanment may compensate forthe excess fuel injected over the preceding enrichment. As such, theamount of excess oxygen may be computed as an area 914 under the curveof the enleanment. That is, area 914 may be an integral of the excessoxygen from the enleanment operation. Thus, the second duration 910 maybe extended based on the exhaust gas content such that the cylinder iscontinued to be enleaned until the exhaust gas oxygen content isreturned above a threshold.

At t3, after the second duration 910 has elapsed, and the exhaust gasoxygen content has been restored above the threshold value, the cylindermay be transitioned back to an air-to-fuel ratio substantially atstoichiometry.

While the depicted example indicates modifying the amount of fuelinjected in the affected cylinder, it will be appreciated that inalternate examples, the amount of fuel injected into all the cylindersof the engine may be adjusted in response to an indication ofpre-ignition in one of the cylinders. As previously elaborated, theenrichment of each cylinder may be adjusted on an individual cylinder,or group of cylinders, basis.

In this way, a rich fuel injection operation may be used to rapidlyaddress pre-ignition while a subsequent lean fuel injection operation isused to address potential catalyst efficiency degradation arising fromthe rich fuel injection operation. Specifically, by balancing the excessfuel from the rich fuel injection operation with the excess oxygen fromthe lean fuel injection operation, exhaust feed-gas oxygen levels may bereturned to within a desired range, thereby also restoring the catalyticefficiency of emission catalysts. By limiting an engine load whileenriching the cylinder, additional cylinder cooling benefits topre-ignition mitigation may be attained. Further, by adjusting theenrichment and the engine load limiting based on a likelihood ofpre-ignition, pre-ignition feedback, and pre-ignition history,pre-ignition may be better anticipated and addressed, thereby reducingengine degradation.

Note that the example control and estimation routines included hereincan be used with various system configurations. The specific routinesdescribed herein may represent one or more of any number of processingstrategies such as event-driven, interrupt-driven, multi-tasking,multi-threading, and the like. As such, various actions, operations, orfunctions illustrated may be performed in the sequence illustrated, inparallel, or in some cases omitted. Likewise, the order of processing isnot necessarily required to achieve the features and advantages of theexample embodiments described herein, but is provided for ease ofillustration and description. One or more of the illustrated actions,functions, or operations may be repeatedly performed depending on theparticular strategy being used. Further, the described operations,functions, and/or acts may graphically represent code to be programmedinto computer readable storage medium in the control system

It will be appreciated that the configurations and routines disclosedherein are exemplary in nature, and that these specific embodiments arenot to be considered in a limiting sense, because numerous variationsare possible. For example, the above technology can be applied to V-6,I-4, I-6, V-12, opposed 4, and other engine types. The subject matter ofthe present disclosure includes all novel and nonobvious combinationsand subcombinations of the various systems and configurations, and otherfeatures, functions, and/or properties disclosed herein.

The following claims particularly point out certain combinations andsubcombinations regarded as novel and nonobvious. These claims may referto “an” element or “a first” element or the equivalent thereof. Suchclaims should be understood to include incorporation of one or more suchelements, neither requiring nor excluding two or more such elements.Other combinations and subcombinations of the disclosed features,functions, elements, and/or properties may be claimed through amendmentof the present claims or through presentation of new claims in this or arelated application. Such claims, whether broader, narrower, equal, ordifferent in scope to the original claims, also are regarded as includedwithin the subject matter of the present disclosure.

1. A method for an engine, comprising, enriching an engine cylinder andlimiting engine load in response to a trip pre-ignition count over adrive cycle and a lifetime pre-ignition count.
 2. The method of claim 1wherein the trip pre-ignition count includes a number of pre-ignitionevents in the cylinder over the drive cycle.
 3. The method of claim 1wherein the trip pre-ignition count includes a number of pre-ignitionevents in the engine over the drive cycle.
 4. The method of claim 1wherein the lifetime pre-ignition count includes a number ofpre-ignition events in the cylinder over a life of the engine.
 5. Themethod of claim 1 wherein the trip pre-ignition count includes a numberof pre-ignition events in the engine over a life of the engine.
 6. Themethod of claim 1 wherein limiting the engine load includes a limitingthe load to a first level responsive to the trip pre-ignition count, andlimiting the load to a second level responsive to the lifetimepre-ignition count.
 7. The method of claim 1, the limiting of the engineload based on whether the pre-ignition is persistent or intermittent,persistent pre-ignition based on a higher number of consecutivepre-ignition events of a plurality of consecutive cylinder combustionevent than the intermittent pre-ignition, the intermittent pre-ignitionincluding a plurality of discrete pre-ignition events over a pluralityof consecutive cylinder combustion events; and the persistentpre-ignition including a plurality of continuous pre-ignition eventsover the plurality of consecutive cylinder combustion events.
 8. Themethod of claim 1 wherein the lifetime pre-ignition count is based ontotal mileage of the vehicle over the lifetime of the vehicle.
 9. Themethod of claim 1 wherein the trip pre-ignition count is based on totalmileage of the vehicle over the drive cycle.
 10. The method of claim 1,wherein limiting engine load includes reducing boost.
 11. The method ofclaim 1, wherein limiting engine load includes reducing throttleopening.
 12. The method of claim 1, wherein limiting engine loadincludes adjusting camshaft timing.
 13. The method of claim 1, wherein adegree of richness of the enrichment is adjusted responsive to a numberof consecutive pre-ignition events in the cylinder, the adjustmentincluding, increasing the degree of richness as the number ofconsecutive pre-ignition events in the cylinder exceeds a threshold. 14.The method of claim 7, further comprising, in response to intermittentor persistent pre-ignition, after enriching the cylinder, enleaning thecylinder, the enleanment based on the preceding enrichment.
 15. A methodfor an engine, comprising, boosting engine intake air; direct injectingfuel into a cylinder; spark-igniting combustion; and enriching the fuelinjection and limiting engine load of the cylinder in response to a trippre-ignition count over a drive cycle and a lifetime pre-ignition count.16. A method for an engine, comprising, boosting engine intake air;direct injecting fuel into a cylinder; spark-igniting combustion; andenriching the fuel injection and limiting engine load of the cylinder inresponse to a trip pre-ignition count over a drive cycle and a lifetimepre-ignition count, and in response to a persistence of thepre-ignition.